MDL-78427 user: Revert accidental addition of deleted line

In MDL-80271, the line `$user->timemodified = time();` was removed to
make sure that `user_update_user()` does not unnecessarily writes to
the database. This seems to have been accidentally put back during
conflict resolution while rebasing the patch for MDL-78427.

user/lib.php

@@ -216,8 +216,6 @@ function user_update_user($user, $updatepassword = true, $triggerevent = true) {
         }
     }
 
-    $user->timemodified = time();
-
     // Validate user data object.
     $uservalidation = core_user::validate($user);
     if ($uservalidation !== true) {